Physicians are increasingly performing catheter-based interventional procedures to treat common vascular diseases in which vessels are narrowed or occluded. Untreated, these conditions can lead to strokes, heart attacks, hypertension, or limb-threatening ischemia depending on the vessel involved. Despite physicians' intention to re-establish blood flow to vital organs, their interventional methods are frequently complicated by the liberation of unwanted material (e.g. clot, fat, and other debris). This results in embolization (blockage of distant blood vessels) and subsequent permanent end-organ damage, often with costly clinical consequences. Adequate distal embolic protection would ensure the safety of these important procedures, and is something physicians are in desperate need of. The prior art teaches several different solutions to the problem of distal protection that could generally be categorized as balloon occlusion devices, filtration devices and flow reversal devices. The following provides a brief description of these devices and their shortcomings.
Balloon Occlusion Devices
PercuSurge, Inc. was the first to recognize this clinical need and addressed it with a low-profile distal balloon system called the GuardWire. It is currently the only distal protection device approved by the FDA and, although only FDA-approved for cardiac applications, it is being routinely used off-label in carotid and other interventions. It requires the passage of a special hollow GuardWire past the narrowing and inflation of an occlusive balloon distal to the lesion. The balloon obstructs normal blood flow through the vessel, creating a barrier for debris, which can be suctioned away through a proximal suction catheter. Although physicians are pleased with its reduction in embolization rates, they still tend to be nervous about using the GuardWire because distal blood flow is completely stopped for the duration of the procedure. Especially in the brain and heart, this occlusion can be very poorly tolerated. With time, the distal tissues die, and thus physicians feel tremendously pressured for time while using this device. Further, it has recently been shown that the initial passage of the GuardWire across the unprotected lesion results in liberation of significant amounts of debris (See e.g. Orlandi et al. in a paper entitled “Characteristics of cerebral microembolism during carotid stenting and angioplasty alone”, and published in Archives of Neurology, Vol. 58(9), September 2001, 1410-1413).
Filtration Devices
In an attempt to better preserve distal flow during protection, a generation of filter devices emerged which are currently in clinical trials. AngioGuard and FilterWire are large-profile umbrella-like devices, which are inserted (again past an unprotected lesion) into place, allowing blood to flow through the filters but trapping small debris, which cannot pass through its pores. Originally the devices were very bulky with 100 μm pores. Later versions have decreased the device profile and pore size down to 80 μm, decreasing the size of particles that are able to embolize through the filter. These filters, however, have several shortcomings in that (1) it is often difficult to get a large filter past a narrow occlusion, (2) there is no distal protection while the guidewire and filter are being inserted across the lesion, (3) they can fill with debris and lead to complete occlusion, (4) their metal edges can traumatize the vessel and cause vascular spasm, dissection, or perforation (5) small debris (<80 μm, which can cause significant end organ damage) can still pass through these filters (See e.g. Rapp et al. in a paper entitled “Atheroemboli to the brain: size threshold for causing acute neuronal cell death” and published in Journal of Vascular Surgery, Vol. 32(1): 68-76, July 2000), and (6) snug apposition of the filter against the vessel wall is difficult to ensure and thus a channel often persists for unprotected flow of debris to the distal vessels. It is of interest to note that animal studies have shown that debris as small as 15-40 μm can cause clinically significant embolization. Although the filter companies are aggressively trying to address these concerns, many of them are inherent to the filter concept, and an optimal solution with these devices is thought to be unlikely.
Flow Reversal Devices
Most recently, ArteriA, Inc. proposed a flow-reversal method, which employs an elaborate system of balloons and catheters to reverse the flow of blood across the lesion so that any debris that is liberated during the procedure flows directly into an external catheter where it can be filtered before being returned to the body via the venous system. While initially embraced in concept for use in the carotid/cerebral circulation, this idea has since met with many technical hurdles. First, only a fraction of patients are candidates for safe flow reversal, which requires an intact Circle of Willis (a highly anastomotic connection of arteries supplying the brain, which is notoriously variable in humans, and is only determined at the time of the procedure by cerebral arteriogram). Second, an elaborate system of catheters must be set up, including an extra-corporeal bypass tract which routes reversed blood back into the body after debris has been filtered out. This cumbersome setup is believed to limit the acceptance of this technology. Further, for the many stenotic lesions that occur in the common carotid artery, this technique would require withdrawal of a balloon past a fully deployed stent. Thus, this limits application of this technique only to stenoses entirely within the internal carotid artery, and not those that extend thru the carotid bifurcation into the common carotid artery.
Accordingly, there is a need to develop new and more effective solutions that can be used for catheter-based interventional procedures and desirably protect against embolization.